In the processing of raw coal, it is generally considered beneficial to reduce the moisture content of low-rank coals (e.g., lignite and subbituminous coals) prior to shipment to the customer. Such reduced moisture content upgrades the low-rank coal to enhance the coal product heating value and decreases the weight of the coal to decrease transportation costs.
Removal of surface moisture and interstitial moisture from low-rank coals has the undesirable effect of increasing particle friability and dustiness during handling. Some dustiness also occurs due to loss of surface moisture by natural means during mining, preparation and storage. The presence of dust is undesirable in that it impairs visibility, is harmful if inhaled for long periods of time, and can result in loss of coal product. More significantly, dustiness can also result in spontaneous combustion during shipment and/or storage of the coal product.
Dust suppressants have been developed for reducing the dustiness of coals and thereby reducing the incidence of spontaneous combustion. Some dust suppressants are also designed to provide a relatively moisture repellant coating on the coal product, thereby inhibiting moisture reabsorption into the dry coal product which would reduce the product heating value. For example, petroleum-based fluids have been applied to coal particles (i.e., sprayed or immersed) to reduce the dustiness of and provide a moisture repellant coating to the coal product. However, petroleum-based fluids can be expensive, typically have high sulfur content, and can have low viscosities resulting in run-off. Molasses has also been utilized as a dust suppressant. Molasses is readily available and is low in cost, but lacks sufficient moisture repellency to be considered alone as a high-grade coal dust suppressant.
Based upon the foregoing, there is a need for an improved dust suppressant which can be economically applied to coal products to inhibit dust formation and improve moisture repellency of the coal product. The composition should have a high static viscosity at ambient temperature to inhibit run-off of the composition during long-term storage of the coal product. Further, the viscosity of the composition should substantially decrease with increasing temperature and increasing shear rate to facilitate application of the composition to the coal product.